Leather-like sheets have been hitherto used in a variety of applications such as interiors, clothes, shoes, briefcases, gloves and upholstery materials for vehicle seats.
In particular, in the field of the upholstery materials for vehicle seats, such as railroad coach seats, automobile seats, airplane seats and ship seats, and for interiors such as cushion sheets, couches and chairs, there are strong demands for materials, especially suede-finished leather-like sheet materials which not only have a soft hand and a beautiful surface appearance but also exhibit various types of fastness, durability and wear resistance as well as excellent flame retardancy.
Hitherto, as a method for imparting flame retardancy to a leather-like sheet comprising a fibrous sheet as a base material, generally employed are a method in which a phosphorus compound is incorporated into the fibers which constitute the base material and a method in which fine particles of a flame retardant, such as a halogen compound containing chlorine or bromine as its main component or an antimony compound, are deposited with a binder on a back surface of the base material. In the case of a leather-like sheet using a nonwoven fabric of ultrafine fibers as a base material, the main trend has been toward the use of the latter method because the strength of ultrafine fibers is deteriorated when the former method is adopted.
In the case of the latter method, however, hazard of toxicity and generation of harmful substances during combustion has been pointed out. There is, therefore, a worldwide tendency to considerably limit the use of the latter method.
As regards a method for imparting flame retardancy to a suede-finished leather-like sheet, a method for back-coating with a flame retardant is disclosed for the development of an upholstery material for vehicle seats or airplane seats (for example, refer to Patent Documents 1 and 2).
With this method, however, the obtained leather-like sheet tends to give a hard hand and lack a high-quality appearance. Further, since the elongation is restricted, it tends to be difficult for the sheet to flexibly follow a complicated shape.
Disclosed also are a method for impregnating and applying a flame retardant in a finishing step of dyeing and a method of admixing a flame retardant to polyurethane (for example, refer to Patent Document 3).
With these method, however, it is necessary to deposit the flame retardant in a large amount in order to achieve flame retarding performance. Therefore, a surface stickiness and a reduction of hand occasionally occur.
Further, in the case of the latter method, the polyurethane resin tends to become poor in light fastness and, therefore, cannot be satisfactorily used as a sheet for an upholstery material of vehicle seats requiring particularly high light fastness.
In general, with a method in which an additive such as a flame retardant is incorporated into polyurethane, it is difficult to achieve both flame retarding performance and the properties inherent to the resin at the same time.
There is also disclosed technique in which elution of a flame retardant and deterioration of fiber physical properties during dyeing are overcome by copolymerizing a phosphorus-based flame retardant with thermoplastic synthetic fibers (for example, refer to Patent Document 4).
With this method, however, it is difficult to achieve both high flame retardancy and reduced cost at the same time because there is a restriction on the proportion of the fibers copolymerized with the phosphorus-based flame retardant due to cost problems.
As a method for imparting flame retardancy to a polyester-based fiber structure, it is also known to exhaust a flame retardant into the fiber in a bath for dyeing (for example, refer to Patent Document 5).
With this method, however, it is necessary to treat the polyester in a hot water at a temperature as high as 130° C. in order to dye the polyester. As a consequence, the flame retardant and a surfactant used for dispersing the flame retardant are apt to be decomposed and denaturated to cause fouling of inside portions of the dyeing apparatus and to reduce the productivity.    Patent Document 1: JP H03-80914B    Patent Document 2: JP H05-302273B    Patent Document 3: JP H07-18584A    Patent Document 4: JP 2004-169197A    Patent Document 5: JP 2004-1316035A